One transferred plasma arc torch according to the prior art is shown in FIG. 1. This transferred plasma arc torch of the prior art has its torch a tapered at its portion facing an electrode c which is held by an electrode holder b. The working gases in the torch are guided to pass through a working gas passage e, which is in the shortest gap around the electrode holder b, and are then accelerated in the nozzle a having a gradually reduced effective area, until they reach the throat f of the nozzle a, which has a diameter d and a length l. From this throat f, the gases are discharged to the outside at a velocity equal to their sonic velocity.
In the case of the transferred plasma arc torch of a metal plate using the aforementioned nozzle a of the prior art, the cut face is highly frequently inclined from the start of the cutting even if the nozzle a used is new.
According to our investigations, it has been found that the major causes for the aforementioned phenomena are, as follows:
Due to a minute displacement between the center lines of the electrode c and the nozzle a, the gap between the electrode c and the nozzle a becomes uneven so that the flow velocity of the working gases passing the working gas passage e in that gap becomes asymmetrical with respect to the center line of the nozzle a to establish serious velocity differences. The working gases having such asymmetrical velocity distribution will establish disturbances such as vortexes when they flow into the mixing chamber located downstream of the electrode c. These disturbances are not damped but released as they are to the outside because they are abruptly accelerated to the exit of the nozzle a having the gradually reduced effect area.
As a result, the velocity distribution remains asymmetrical with respect to the center line of the nozzle a even at the throat f of the nozzle a so that the directivity of the arc jet discharged to the atmosphere is deteriorated to induce a deflection of the arc jet.
In order to examine these phenomena experimentally, we have observed the behavior of the arc jet g to be discharged from the nozzle a, by grinding out a portion of the tip of the electrode holder b of the commercially available air plasma arc torch of 50 A grade to enlarge the difference in the working gas velocity which is established by the working gas passage e in the gap between the nozzle a and the electrode holder b.
These experiments have been performed for several electrode holders and have confirmed that the direction of deflection of the arc jet g and the direction of the ground-out portion of the electrode holder b are aligned.
This means that the asymmetry, if any, of the gap to be formed between the nozzle a and the electrode holder b due to the offset of the axes will induce the deflection of the arc jet g thereby to cause an inclination of an angle .theta. at the cut face.
The aforementioned inclination of the cut face grows a serious problem in case the thickness of the plate to be cut is enlarged. In the case of cutting a thick plate, a secondary working may be required, or still the worse the cut work cannot be used as the product, if the angle of inclination of the cut face exceeds some extent.
In order to prevent occurrence of the aforementioned inclination of the cut face, it has been endeavored to raise the working accuracy of a component such as the arc body, the electrode holder or the nozzle. Despite of this endeavor, however, a complete solution has not been obtained yet due to the practical limits in the working accuracy and the assembling accuracy.